Though the earliest theories about atoms were proposed by Democritus and Aristotle, the first accepted theory was proposed by John Dalton. John Dalton (1766 - 1844), an English chemist, physicist, and meteorologist, is honored mainly because of his contribution to modern atomic theory and color blindness. It was the research studies on properties of atmosphere and gases in 1803, which made him realize about the particles (later called atoms) and their weight. John Dalton's Initial Work John Dalton had a strong interest in meteorology, and did extensive studies regarding nature of atmospheric gases. He found out the formation of compounds by combining oxygen and carbon dioxide. Later, John Dalton published the first table on relative atomic weights that included six elements, viz., hydrogen, oxygen, carbon, nitrogen, phosphorous, and sulfur. He stated the atomic weight of hydrogen to be 1. It was not confirmed as to how he derived the atomic weights. In Dalton's laboratory notebook (dated September 1803), he indicated that the relative weights of the gaseous elements could be found out from the studies of other compounds, such as water, carbon dioxide, and ammonia. In his notebook, the elements were represented by symbols. He was the first person to propose the idea of identifying an element by its symbol. Much later, elements started to be represented by their abbreviations. For example, 'H' is the abbreviation of the element hydrogen. The same research paper was published in 1805. Atomic Theory by John Dalton The existence of atoms had been mentioned by many other pioneers before John Dalton actually put forth the atomic theory. However, there was no elaboration on the atomic weight part. As per the lecture given by John Dalton in 1803 in the Royal Institution (London), atoms of one element differed from those of other elements by their relative weights. Following are some of the important points proposed by John Dalton in his atomic theory.

• All elements are made up of tiny indivisible particles, known as atoms.
• Atoms of the same element are identical with respect to their size, weight, and properties.
• Atoms of different elements are different from each other and can be identified by their relative weights.
• Atoms can neither be divided into smaller particles nor destroyed.
• Chemical reactions occur due to the rearrangement, combination, or separation of the atoms.
• Atoms combine in the ratio of whole numbers such as 1:1, 1:2, 2:3, etc.
• Atoms of two or more different elements combine together to form chemical compounds

In 1808, John Dalton published a list of elements along with their atomic weights in his 'New System of Chemical Philosophy', for which he received the Royal Medal in 1826. He referred the masses of elements as weights. Since then, chemists used either 'atomic weight' or 'atomic mass'. In spite of the fact that there are some flaws in his work (for example, he had assigned the atomic weight of oxygen as 7), John Dalton was the pioneer who had given a pathbreaking view to future experiments. In order to honor his work, scientists referred one atomic mass unit as 'Dalton' (abbreviated as Da) for many years. In 1886, Eugen Goldstein, a German physicist, discovered positive charged particles in atoms. Later, J. J. Thomson, a British physicist, discovered the electron by using a cathode ray tube in 1897. In contrary to John Dalton's assumptions that atoms are indivisible, he suggested that atoms are made up of smaller particles. In 1911, Rutherford, a student of Thomson, put forth the view that an atom consisted of a dense positive core at the center (nucleus), surrounded by electrons. He named the positive particles in the nucleus as protons, and stated that protons were larger than the electrons and carried an equal and opposite charge to electrons. In the year 1932, James Chadwick discovered the electrically neutral particle in the atom and named it as the neutron. In John Dalton's atomic model, the concept of isotopes of an element is not mentioned. They represent different types of atoms of an element, which have different atomic masses but possess similar properties. Also, in contrary to what Dalton had stated earlier, destroying atoms is possible with the help of nuclear reactions. This way, the modern atomic theory was expounded in the later years.